Silicon carbide substrate, semiconductor device, and methods for manufacturing them

ABSTRACT

A silicon carbide substrate has a first main surface, and a second main surface opposite to the first main surface. A region including at least one main surface of the first and second main surfaces is made of single-crystal silicon carbide. In the one main surface, sulfur atoms are present at not less than 60×10 10  atoms/cm 2  and not more than 2000×10 10  atoms/cm 2 , and carbon atoms as an impurity are present at not less than 3 at % and not more than 25 at %. Thereby, a silicon carbide substrate having a stable surface, a semiconductor device using the substrate, and methods for manufacturing them can be provided.

This application claims the benefit of U.S. Provisional Application No.61/618,928 filed Apr. 2, 2012, which is incorporated by reference hereinin the entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon carbide substrate, asemiconductor device, and methods for manufacturing them. Moreparticularly, the present invention relates to a silicon carbidesubstrate capable of improving yield of semiconductor devicecharacteristics, a semiconductor device using the substrate, and methodsfor manufacturing them.

2. Description of the Background Art

In recent years, in order to achieve higher breakdown voltage and lowerloss of a semiconductor device, use thereof in a high temperatureenvironment, and the like, silicon carbide has increasingly been adoptedas a material for forming a semiconductor device. Since silicon carbideis excellent in heat conductivity when compared with a semiconductormade of a nitride such as gallium nitride, it is excellent as asubstrate for a semiconductor device for high-power application at highvoltage and large current.

In order to form a high-quality epitaxial layer on a substrate, thesubstrate may be subjected to surface treatment before formation of theepitaxial layer. For example, Japanese Patent Laying-Open No.2010-163307 describes a nitride substrate having a surface layercontaining 3 atomic percent (at %) to 25 at % of carbon and 5×10¹⁰atoms/cm² to 200×10¹⁰ atoms/cm² of a p type metal element. Thereby, anitride substrate having a stabilized surface is obtained.

However, the state of a surface differs depending on the substratematerial, resulting in differences in surface oxidation and easiness ofadsorption or adherence of an impurity to the surface. Therefore, evenif the method described in Japanese Patent Laying-Open No. 2010-163307is applied to a silicon carbide substrate, it is difficult to obtain asilicon carbide substrate having a stabilized surface.

SUMMARY OF THE INVENTION

The present invention has been made to solve such a problem, and oneobject of the present invention is to provide a silicon carbidesubstrate having a stabilized surface, a semiconductor device using thesubstrate, and methods for manufacturing them.

The inventor of the present invention has studied the relationshipbetween the state of a surface of a silicon carbide substrate andcharacteristics of a semiconductor device using the substrate. As aresult, the inventor has found that the characteristics of thesemiconductor device are influenced by the presence of an impurityelement in a main surface of the silicon carbide substrate on which anepitaxial growth layer is to be formed. The inventor has also obtained afinding that the characteristics of the semiconductor device differdepending on the type of the impurity element.

Specifically, when there are many impurities in the surface of thesilicon carbide substrate, epitaxial growth is inhibited because latticematching with the substrate is hindered. Further, when a native oxidefilm is formed on the surface of the silicon carbide substrate fromoxygen in an atmosphere, the quality of an epitaxial layer grown bylattice matching with the substrate is deteriorated. In addition,silicon (Si) as an impurity from the atmosphere is likely to adhere tothe surface of the silicon carbide substrate. When silicon is increased,a piled-up layer is formed at an interface between the silicon carbidesubstrate and the epitaxial layer, reducing a resistance at theinterface. When the resistance at the interface is reduced, a currentleaks toward the substrate, causing deterioration in the characteristicsof the semiconductor device. In particular, the deterioration in thecharacteristics of the semiconductor device due to the leakage currentis more significant in a lateral semiconductor device.

As a result of earnest study, the inventor has obtained the followingfindings. Due to the presence of certain amounts of sulfur atoms andcarbon atoms as an impurity in the surface of the silicon carbidesubstrate, oxidation of the surface and an increase in impurities aresuppressed, and thereby deterioration in the quality of the epitaxiallayer formed on the surface can be suppressed. In addition, due to thepresence of certain amounts of sulfur atoms and carbon atoms as animpurity in the surface of the silicon carbide substrate, silicon as animpurity is suppressed from adhering to the surface of the siliconcarbide substrate. Thus, a reduction in the resistance at the interfacebetween the silicon carbide substrate and the epitaxial layer can besuppressed. As a result, yield of a semiconductor device manufacturedusing the silicon carbide substrate can be improved.

As described above, it has been found that, due to the presence ofcertain amounts of sulfur atoms and carbon atoms as an impurity in thesurface of the silicon carbide substrate, the surface of the siliconcarbide substrate is stabilized, and yield of a semiconductor deviceformed using the substrate can be improved.

A silicon carbide substrate in accordance with the present invention hasa first main surface, and a second main surface opposite to the firstmain surface. A region including at least one main surface of the firstand second main surfaces is made of single-crystal silicon carbide.Sulfur atoms are present in the one main surface at not less than60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and carbonatoms as an impurity are present in the one main surface at not lessthan 3 at % and not more than 25 at %.

According to the silicon carbide substrate in accordance with thepresent invention, the presence ratio of the sulfur atoms in the onesurface is not less than 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰atoms/cm², and the presence ratio of the carbon atoms as the impurity inthe one surface is not less than 3 at % and not more than 25 at %. Sincethe presence ratio of the sulfur atoms is not less than 60×10¹⁰atoms/cm² and the presence ratio of the carbon atoms as the impurity isnot less than 3 at %, the silicon carbide substrate has a stabilizedsurface. In addition, since the presence ratio of the sulfur atoms isnot more than 2000×10¹⁰ atoms/cm² and the presence ratio of the carbonatoms as the impurity is not more than 25 at %, inhibition of epitaxialgrowth due to hindered lattice matching with the substrate can besuppressed. As a result, a silicon carbide substrate capable ofimproving yield of a semiconductor device can be provided.

It is noted that the presence ratio of the sulfur atoms in the mainsurface of the silicon carbide substrate can be measured for example byTXRF (Total Reflection X-Ray Fluorescence) or the like. Further, thepresence ratio of the carbon atoms in the main surface of the siliconcarbide substrate can be measured by AES (Auger Electron Spectroscopy),XPS (X-ray Photoelectron Spectroscopy), or the like. In the measurementby these analysis methods, the presence ratio of an impurity element ismeasured based on information in a region from the main surface to adepth of about 5 nm. That is, in the present application, the presenceratio of an element in the main surface of the silicon carbide substratemeans the presence ratio of the element in a region from the mainsurface to a depth of about 5 nm. It is noted that, since XPS evaluatesbinding energy, it can separately evaluate carbon constituting siliconcarbide and carbon contained in an organic substance and the likeadhering to a surface, that is, carbon as an impurity. Specifically, byobserving peak shift (chemical shift) in XPS spectra, carbon detected atabout 281 to 283 eV is determined as carbon of Si—C, and carbon detectedat about 284 to 293 eV is determined as carbon as an impurity adheringto a surface of a substrate.

The carbon atoms as the impurity refer to, for example, carbon atomsadhering to the silicon carbide substrate, as an impurity. In otherwords, the carbon atoms as the impurity refer to carbon atoms not havingsilicon carbide binding. Examples of the carbon atoms as the impurityinclude carbon atoms in a bound state such as C—C, C—H, C═C, C—OH, orO═C—OH.

Preferably, in the silicon carbide substrate, chlorine atoms are presentin the one main surface at not more than 3000×10¹⁰ atoms/cm². Thereby,deterioration in the quality of an epitaxial growth layer can besuppressed. Further, the silicon carbide substrate has a stabilizedsurface. As a result, a silicon carbide substrate capable of improvingyield of a semiconductor device can be provided.

It is noted that the presence ratio of the chlorine atoms in the mainsurface of the silicon carbide substrate can be measured for example byTXRF or the like.

Preferably, in the silicon carbide substrate, oxygen atoms are presentin the one main surface at not less than 3 at % and not more than 30 at%. Since the presence ratio of the oxygen atoms is not less than 3 at %,the silicon carbide substrate has a stabilized surface. Further, sincethe presence ratio of the oxygen atoms is not more than 30 at %,deterioration in the quality of the epitaxial growth layer can besuppressed. As a result, a silicon carbide substrate capable ofimproving yield of a semiconductor device can be provided.

It is noted that the presence ratio of the oxygen atoms in the mainsurface of the silicon carbide substrate can be measured by AES, XPS, orthe like.

Preferably, in the silicon carbide substrate, a metal impurity ispresent in the one main surface at not more than 4000×10¹⁰ atoms/cm².Thereby, deterioration in the quality of the epitaxial growth layer canbe suppressed. Further, the silicon carbide substrate has a stabilizedsurface. As a result, a silicon carbide substrate capable of improvingyield of a semiconductor device can be provided.

It is noted that the presence ratio of the metal impurity in the mainsurface of the silicon carbide substrate can be measured for example byTXRF or the like.

Preferably, in the silicon carbide substrate, the one main surface has asurface roughness of not more than 0.5 nm when evaluated in aroot-mean-square roughness Rq (see the Japanese Industrial Standards:JIS). This allows easy formation of a good-quality epitaxial growthlayer on the one main surface. As a result, a silicon carbide substratecapable of improving yield of a semiconductor device can be provided.

It is noted that the surface roughness of the main surface can bemeasured for example with an AFM (Atomic Force Microscope), an opticalinterference-type roughness meter, a stylus-type roughness meter, or thelike.

Preferably, the silicon carbide substrate has a diameter of not lessthan 110 mm. In a process for manufacturing a semiconductor device usingsuch a large-diameter substrate, manufacturing efficiency of thesemiconductor device can be improved, and manufacturing cost thereof canbe suppressed.

Preferably, the silicon carbide substrate has a diameter of not lessthan 125 mm and not more than 300 mm. From the viewpoint of improvingproductivity, a large-area substrate having a diameter of approximately125 mm or more is desired. If the substrate has a diameter of more than300 mm, in-plane distribution of the surface impurities is increased.Further, advanced control is required to suppress warpage of thesubstrate. Thus, it is desirable that the silicon carbide substrate hasa diameter of not more than 300 mm.

Preferably, in the silicon carbide substrate, the single-crystal siliconcarbide has a 4H structure. The one main surface has an off angle of notless than 0.1° and not more than 10° relative to a {0001} plane of thesingle-crystal silicon carbide.

Silicon carbide having a 4H structure, which is hexagonal siliconcarbide, can be efficiently grown by being grown in a <0001> direction.In addition, a substrate having a small off angle, specifically, an offangle of not more than 10°, relative to the {0001} plane can beefficiently fabricated from a crystal grown in the <0001> direction. Onthe other hand, good epitaxial growth is easily performed by providingthe one main surface with an off angle of not less than 0.1° relative tothe {0001} plane.

Preferably, in the silicon carbide substrate, the single-crystal siliconcarbide has a 4H structure. The one main surface has an off angle of notmore than 4° relative to a {03-38} plane of the single-crystal siliconcarbide.

Thereby, suppression of leakage current, improvement in channelmobility, and the like in a semiconductor device manufactured using thesubstrate are easily achieved.

The silicon carbide substrate has a base layer, and a single-crystalsilicon carbide layer formed on the base layer. The one main surface isa surface of the single-crystal silicon carbide layer on a side oppositeto a side facing the base layer.

Thereby, a silicon carbide substrate can be manufactured relativelyinexpensively for example by preparing an inexpensive base substrate,specifically, a substrate made of single-crystal silicon carbide havinga high defect density, a polycrystalline silicon carbide substrate, or abase substrate made of ceramics, as a base layer, and arranging asubstrate made of a good-quality silicon carbide single crystal on thebase substrate. In particular, since it is difficult to obtain alarge-diameter silicon carbide substrate, an inexpensive, large-diametersilicon carbide substrate can be obtained for example by arranging aplurality of single-crystal silicon carbide substrates which have goodquality but are small in size on a base substrate, side by side whenviewed in plan view, to fabricate a silicon carbide substrate having aplurality of single-crystal silicon carbide layers arranged side by sideon a base layer along a main surface of the base layer.

A semiconductor device in accordance with the present invention has asilicon carbide substrate, an epitaxial growth layer, and an electrode.The silicon carbide substrate has a first main surface, and a secondmain surface opposite to the first main surface. A region including atleast one main surface of the first and second main surfaces of thesilicon carbide substrate is made of single-crystal silicon carbide.Sulfur atoms are present in the one main surface at not less than60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and carbonatoms as an impurity are present in the one main surface at not lessthan 3 at % and not more than 25 at %. The epitaxial growth layer isformed on the one main surface of the silicon carbide substrate. Theelectrode is formed on the epitaxial growth layer.

According to the semiconductor device in accordance with the presentinvention, since the presence ratio of the sulfur atoms is not less than60×10¹⁰ atoms/cm² and the presence ratio of the carbon atoms as theimpurity is not less than 3 at %, the silicon carbide substrate has astabilized surface. In addition, since the presence ratio of the sulfuratoms is not more than 2000×10¹⁰ atoms/cm² and the presence ratio of thecarbon atoms as the impurity is not more than 25 at %, inhibition ofepitaxial growth due to hindered lattice matching with the substrate canbe suppressed. As a result, yield of the semiconductor device can beimproved.

A method for manufacturing a silicon carbide substrate in accordancewith the present invention includes the steps of: preparing a crystal ofsingle-crystal silicon carbide; obtaining a substrate having a firstmain surface and a second main surface opposite to the first mainsurface, by cutting the crystal; planarizing at least one main surfaceof the first and second main surfaces; and performing finishingtreatment on a planarized surface of the substrate. In the step ofperforming finishing treatment on the surface of the substrate, presenceratios of sulfur atoms and carbon atoms as an impurity in the one mainsurface are adjusted such that the sulfur atoms are present in the onemain surface at not less than 60×10¹⁰ atoms/cm² and not more than2000×10¹⁰ atoms/cm², and the carbon atoms as the impurity are present inthe one main surface at not less than 3 at % and not more than 25 at %.

According to the method for manufacturing a silicon carbide substrate inaccordance with the present invention, a silicon carbide substratecapable of improving yield of a semiconductor device can be provided.

A method for manufacturing a semiconductor device in accordance with thepresent invention includes the steps of: preparing a silicon carbidesubstrate including a first main surface and a second main surfaceopposite to the first main surface, a region including at least one mainsurface of the first and second main surfaces of the silicon carbidesubstrate being made of single-crystal silicon carbide, sulfur atomsbeing present in the one main surface at not less than 60×10¹⁰ atoms/cm²and not more than 2000×10¹⁰ atoms/cm², and carbon atoms as an impuritybeing present in the one main surface at not less than 3 at % and notmore than 25 at %; forming an epitaxial growth layer on the one mainsurface of the silicon carbide substrate; and forming an electrode onthe epitaxial growth layer.

According to the method for manufacturing a semiconductor device inaccordance with the present invention, yield of the semiconductor devicecan be improved.

As is clear from the above description, according to the silicon carbidesubstrate, the semiconductor device using the substrate, and the methodsfor manufacturing them in accordance with the present invention, asilicon carbide substrate having a stabilized surface, a semiconductordevice using the substrate, and methods for manufacturing them can beprovided.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing a structure of asilicon carbide substrate in a first embodiment.

FIG. 2 is a flowchart schematically showing a method for manufacturingthe silicon carbide substrate in the first embodiment.

FIG. 3 is a schematic perspective view for illustrating the method formanufacturing the silicon carbide substrate in the first embodiment.

FIG. 4 is a schematic plan view for illustrating the method formanufacturing the silicon carbide substrate in the first embodiment.

FIG. 5 is a schematic perspective view for illustrating the method formanufacturing the silicon carbide substrate in the first embodiment.

FIG. 6 is a schematic cross sectional view showing a structure of alateral MESFET in the first embodiment.

FIG. 7 is a flowchart schematically showing a method for manufacturingthe lateral MESFET in the first embodiment.

FIG. 8 is a schematic cross sectional view showing a structure of asilicon carbide substrate in a second embodiment.

FIG. 9 is a flowchart schematically showing a method for manufacturingthe silicon carbide substrate in the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. It is noted that in the below-mentioneddrawings, the same or corresponding portions are given the samereference characters and are not described repeatedly. Further, in thepresent specification, an individual orientation is represented by [ ],a group orientation is represented by < >, an individual plane isrepresented by ( ) and a group plane is represented by { }. In addition,a negative index is supposed to be crystallographically indicated byputting “-” (bar) above a numeral, but is indicated by putting thenegative sign before the numeral in the present specification.

First Embodiment

First, a silicon carbide substrate as one embodiment of the presentinvention will be described. Referring to FIG. 1, a silicon carbidesubstrate 1 in the present embodiment is entirely made of single-crystalsilicon carbide, and has a first main surface 1A and a second mainsurface 1B opposite to the first main surface. In at least one mainsurface of first and second main surfaces 1A and 1B (for example, firstmain surface 1A), sulfur atoms are present at not less than 60×10¹⁰atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and carbon atoms as animpurity are present at not less than 3 at % and not more than 25 at %.

Since the presence ratio of the sulfur atoms is not less than 60×10¹⁰atoms/cm² and the presence ratio of the carbon atoms as the impurity isnot less than 3 at %, the silicon carbide substrate has a stabilizedsurface. In addition, since the presence ratio of the sulfur atoms isnot more than 2000×10¹⁰ atoms/cm² and the presence ratio of the carbonatoms as the impurity is not more than 25 at %, inhibition of epitaxialgrowth due to hindered lattice matching with the substrate can besuppressed. As a result, a silicon carbide substrate capable ofimproving yield of a semiconductor device can be provided.

The amount of the sulfur atoms present in the one main surface ofsilicon carbide substrate 1 is preferably not less than 80×10¹⁰atoms/cm² and not more than 800×10¹⁰ atoms/cm², and more preferably notless than 120×10¹⁰ atoms/cm² and not more than 600×10¹⁰ atoms/cm².

The amount of the carbon atoms as the impurity present in the one mainsurface of silicon carbide substrate 1 is preferably not less than 7 at% and not more than 21 at %, and more preferably not less than 10 at %and not more than 18 at %.

Chlorine atoms are present in the one main surface of silicon carbidesubstrate 1 in accordance with the present embodiment at not more than3000×10¹⁰ atoms/cm². The amount of the chlorine atoms is preferably notmore than 1300×10¹⁰ atoms/cm², and more preferably not more than100×10¹⁰ atoms/cm². Thereby, a silicon carbide substrate capable offurther improving yield of a semiconductor device can be provided.

Oxygen atoms are present in the one main surface of silicon carbidesubstrate 1 in accordance with the present embodiment at not less than 3at % and not more than 30 at %. The amount of the oxygen atoms ispreferably not less than 5 at % and not more than 21 at %, and morepreferably not less than 9 at % and not more than 15 at %. Thereby, asilicon carbide substrate capable of further improving yield of asemiconductor device can be provided.

A metal impurity is present in the one main surface of silicon carbidesubstrate 1 in accordance with the present embodiment at not more than4000×10¹⁰ atoms/cm². The amount of the metal impurity is preferably notmore than 900×10¹⁰ atoms/cm², and more preferably not more than 80×10¹⁰atoms/cm². Examples of the metal impurity include Ti (titanium), Cr(chromium), Fe (iron), Ni (nickel), Cu (copper), Zn (zinc), Ca(calcium), K (potassium), Al (aluminum), and the like. By reducing theamount of the metal impurity, the quality of an epitaxial growth layercan be improved.

The one main surface of silicon carbide substrate 1 in accordance withthe present embodiment has a surface roughness of not more than 0.5 nmwhen evaluated in Rq as a root-mean-square roughness (see the JapaneseIndustrial Standards: JIS). This allows easy formation of a high-qualityepitaxial growth layer on the one main surface of silicon carbidesubstrate 1. As a result, a silicon carbide substrate capable ofimproving yield of a semiconductor device can be provided. Rq ispreferably not more than 0.3 nm, and more preferably not more than 0.1nm.

Preferably, silicon carbide substrate 1 has a diameter of not less than110 mm. Using a large-area substrate leads to an increase in the numberof chips to be obtained. Thereby, cost and productivity in a deviceprocess can be improved. Further, silicon carbide substrate 1 preferablyhas a diameter of not less than 125 mm and not more than 300 mm. Fromthe viewpoint of improving productivity, a large-area substrate isdesired. However, if the substrate has a diameter of more than 300 mm,in-plane distribution of the surface impurities is increased. Further,advanced control is required to suppress warpage of the substrate.

Preferably, the single-crystal silicon carbide forming the substrate hasa 4H structure, and the one main surface has an off angle of not lessthan 0.1° and not more than 10° relative to a {0001} plane. Preferably,the one main surface is a surface that is off from a {000-1} plane by0.01 to 5°.

Preferably, the single-crystal silicon carbide forming the substrate hasa 4H structure, and the one main surface has an off angle of not morethan 4° relative to a {03-38} plane. Preferably, the one main surface isa surface that is off from a {01-11} plane or a {01-12} plane by notmore than 4°, or a surface that is off from a {0-33-8} plane, a {0-11-1}plane, or a {0-11-2} plane by not more than 4°. Thereby, a particularlygood oxide film is obtained, and thus good characteristics are obtainedin a semiconductor device such as a MOSFET (Metal Oxide SemiconductorField Effect Transistor).

Next, a method for manufacturing silicon carbide substrate 1 will bedescribed. Referring to FIG. 2, in the method for manufacturing siliconcarbide substrate 1 in the present embodiment, first, a crystal growthstep is performed as step (S10). In this step (S10), single-crystalsilicon carbide is fabricated, for example, by a sublimation methoddescribed below.

First, a seed crystal made of single-crystal silicon carbide and sourcematerial powder made of silicon carbide are introduced into a containermade of graphite. Subsequently, the source material powder is heated,and thereby silicon carbide is sublimated and recrystallized on the seedcrystal. On this occasion, recrystallization proceeds while a desiredimpurity such as nitrogen is being introduced. Then, heating is stoppedwhen a crystal of a desired size is grown on the seed crystal, and acrystal of single-crystal silicon carbide is taken out of the container.

Next, an ingot shaping step is performed as step (S20). In this step(S20), the crystal of single-crystal silicon carbide fabricated in step(S10) is processed into an ingot 10 having, for example, a cylindricalshape shown in FIG. 3. On this occasion, since growing hexagonal siliconcarbide in the <0001> direction can efficiently promote crystal growthwhile suppressing occurrence of a defect, it is preferable to fabricateingot 10 having a longitudinal direction corresponding to the <0001>direction as shown in FIG. 3.

Next, a slicing step is performed as step (S30). In this step (S30), asubstrate is fabricated by cutting ingot 10 obtained in step (S20).Specifically, referring to FIG. 4, first, fabricated columnar(cylindrical) ingot 10 is set such that a portion of its side surface issupported by a support 20. Subsequently, while a wire 9 is running in adirection along a direction of a diameter of ingot 10, ingot 10approaches wire 9 along a cutting direction α perpendicular to therunning direction, and thus ingot 10 comes into contact with wire 9.Then, ingot 10 keeps moving along cutting direction α, and thereby ingot10 is cut. Thus, silicon carbide substrate 1 shown in FIG. 5 isobtained. On this occasion, ingot 10 is cut such that main surface 1A ofsilicon carbide substrate 1 has a desired plane orientation.

Next, a surface planarization step is performed as step (S40). In thisstep (S40), grinding processing, polishing processing, and the like areperformed on main surface 1A of silicon carbide substrate 1 to reduceroughness of a cut surface formed in step (S30) (i.e., main surface 1A).In the grinding processing, a diamond grindstone is used as a tool, andthe grindstone is set to face silicon carbide substrate 1 and rotated tocut into it at a constant speed, and thereby removes a surface of thesubstrate. Main surface 1A can be planarized by removing its unevenness,and its thickness can be adjusted. In the polishing processing, adesired surface roughness can be obtained by adjusting a grain size ofabrasive grains of diamond or the like. As a surface plate, a surfaceplate made of a metal such as iron, copper, tin, a tin alloy, or thelike, a composite surface plate made of a metal and a resin, or apolishing cloth can be used. Using a hard metal surface plate canimprove a rate. Using a soft surface plate can reduce the surfaceroughness.

Next, a surface finishing step is performed as step (S50). In this step(S50), by performing dry etching, CMP (Chemical Mechanical Polishing),or the like on main surface 1A of silicon carbide substrate 1, theamounts of the sulfur atoms, the carbon atoms as the impurity, thechlorine atoms, the oxygen atoms, and the metal impurity that arepresent on the surface of the silicon carbide substrate, the surfaceroughness of the silicon carbide substrate, and the like can becontrolled to be within desired ranges.

For example, while lapping, polishing, and the like can be used as apolishing method for controlling the surface roughness of the siliconcarbide substrate, it is preferable to use CMP treatment to performfinish polishing in order to reduce the surface roughness and controlsurface composition. Abrasive grains for the CMP are required to be madeof a material softer than silicon carbide in order to reduce the surfaceroughness and a process-damaged layer, and colloidal silica or fumedsilica is preferable. A solution for the CMP preferably has a pH of notmore than 4 or not less than 9.5, and more preferably has a pH of notmore than 2 or not less than 10.5, to increase a chemical actionthereof. The pH of the CMP solution can be controlled by adding: aninorganic acid such as hydrochloric acid, nitric acid, sulfuric acid, orphosphoric acid; an organic acid such as formic acid, acetic acid,oxalic acid, citric acid, malic acid, tartaric acid, succinic acid,phthalic acid, or fumaric acid; an inorganic alkali such as KOH, NaOH,or NH₄OH; an organic alkali such as choline, amine, or TMAH; or a saltthereof. Further, it is preferable to add an oxidant. As the oxidant,hypochlorous acid or a salt thereof, a chlorinated isocyanuric acid suchas trichloroisocyanuric acid, a chlorinated isocyanurate such as sodiumdichloroisocyanurate, a permanganate such as potassium permanganate, adichromate such as potassium dichromate, a bromate such as potassiumbromate, a thiosulfate such as sodium thiosulfate, nitric acid, sulfuricacid, hydrochloric acid, aqueous hydrogen peroxide, ozone, or the likecan be used. The pH can also be controlled by adding the oxidant.

Sulfuric acid or sulfate is preferably used for pH adjustment, becauseit facilitates control of the amount of sulfur present in the surface ofthe silicon carbide substrate. In addition, an organic acid, an organicalkali, or a salt thereof is preferably used for pH adjustment, becauseit facilitates control of the amount of carbon present in the surface ofthe silicon carbide substrate. Examples of the organic acid includecarboxylic acid, and examples of the organic alkali include choline,TMAH (tetramethylammonium hydroxide), and the like.

Aqueous hydrogen peroxide is preferably adopted as the oxidant, becauseit facilitates control of the amount of oxygen present in the surface ofthe silicon carbide substrate. The amount of chlorine present in thesurface of the silicon carbide substrate can be controlled by using achlorine-based oxidant. In order to control the composition of thesurface of the silicon carbide substrate, control the surface roughness,and improve the rate, it is preferable to select x and y to satisfy−50x+700≦y≦−50x+1800, where x represents a pH of the solution, and yrepresents an oxidation-reduction potential. By controlling theoxidation-reduction potential to be within an appropriate range tocontrol oxidation power of the solution, the amount of oxygen in thesurface of the silicon carbide substrate can be controlled, and thesurface roughness and a polishing speed can be controlled to be withinappropriate ranges.

In order to control the composition of the surface of the siliconcarbide substrate, control the surface roughness, and improve the rate,it is preferable to set a resistance coefficient R (m²/s), which isrepresented using a viscosity η (mPa·s) of a polishing liquid, a liquidflow rate Q (m³/s), an area S (m²) of the polishing surface plate, apolishing pressure P (kPa), and a peripheral speed V(m/s) (here,determined as R=η×Q×V/S×P), to 2.0×10⁻¹⁵ to 8.0×10⁻¹⁴. By controllingthe resistance coefficient, a resistance applied to the substrate duringpolishing by friction between the polishing cloth and the substrate canbe controlled. Further, the composition of the surface can beeffectively controlled, and the surface roughness and the polishingspeed can be controlled to be within appropriate ranges.

Regarding polishing of a back surface, it is preferable to finish theback surface by polishing using fine diamond abrasive grains. Althoughthe CMP treatment can reduce surface roughness, there arise problems interms of cost and productivity. The diamond abrasive grains preferablyhave a grain size of 0.1 μm to 3 μm. As a surface plate, a surface platemade of a metal such as tin, a tin alloy, or the like, a resin surfaceplate, or a polishing cloth can be used. Using a metal surface plate canimprove a rate. Using a polishing cloth can reduce the surfaceroughness. In order to set the surface roughness within an appropriaterange, it is preferable to set resistance coefficient R (m²/s) to1.0×10⁻¹⁸ to 3.0×10⁻¹⁷. By controlling the resistance coefficient, aresistance applied to the substrate during polishing by friction betweenthe surface plate and the substrate can be uniformized all over thesubstrate and set within a range appropriate for finishing the surface,and in-plane distribution of the roughness can be reduced. The backsurface is preferably has roughness Rq of 0.3 nm to 10 nm. A goodepitaxial layer can be grown by stabilizing contact with a susceptor touniformize temperature distribution during epitaxial growth, andsuppressing warpage during heating.

Dry etching may be performed to control the amounts of the sulfur atoms,the carbon atoms as the impurity, the chlorine atoms, the oxygen atoms,and the metal impurity that are present on the surface of siliconcarbide substrate 1, the surface roughness of silicon carbide substrate1, and the like to be within desired ranges. For example, the amount ofthe sulfur atoms in the surface of silicon carbide substrate 1 can becontrolled by using a sulfur-based gas such as hydrogen sulfide. Theamount of the carbon atoms as the impurity in the surface of siliconcarbide substrate 1 can be controlled by using a carbon-based gas suchas methane, ethane, propane, or acetylene. The amount of the oxygenatoms in the surface of silicon carbide substrate 1 can be controlled byusing oxygen gas. The amount of the chlorine atoms in the surface ofsilicon carbide substrate 1 can be controlled by using chlorine or achlorine-based gas such as boron trichloride. In addition, the amount ofcarbon can also be controlled by etching and reducing silicon in thesubstrate using a chlorine-based gas or a fluorine-based gas.

Next, a cleaning step is performed as step (S60). In this step (S60),foreign matter adhering to the surface during the process up to step(S50) is removed by cleaning. The presence ratios of atoms such as thesulfur atoms and the carbon atoms in the surface of the silicon carbidesubstrate can be adjusted to be within the desired ranges, throughselection of a chemical solution, application of ultrasound, overflowcirculation of the chemical solution in a cleaning tank, and removal ofparticles using a filter in the cleaning step. As the chemical solution,an inorganic acid, an inorganic alkali, an organic acid, or an organicalkali can be used. An oxidant such as aqueous hydrogen peroxide can beused to enhance cleaning effect. The ultrasound can have a frequency of50 kHz to 2 MHz. The filter for circulating the chemical solutionpreferably has a pore diameter of not less than 50 nm and not more than5 μm. Through the above steps, silicon carbide substrate 1 in thepresent embodiment is completed.

According to the method for manufacturing silicon carbide substrate 1 inthe present embodiment, in the step of performing finishing treatment onthe surface of the substrate, the presence ratios of the sulfur atomsand the carbon atoms as the impurity in the one main surface areadjusted such that the sulfur atoms are present in the one main surfaceat not less than 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰atoms/cm², and the carbon atoms as the impurity are present in the onemain surface at not less than 3 at % and not more than 25 at %. Sincethe presence ratio of the sulfur atoms is not less than 60×10¹⁰atoms/cm² and the presence ratio of the carbon atoms as the impurity isnot less than 3 at %, the silicon carbide substrate has a stabilizedsurface. In addition, since the presence ratio of the sulfur atoms isnot more than 2000×10¹⁰ atoms/cm² and the presence ratio of the carbonatoms as the impurity is not more than 25 at %, inhibition of epitaxialgrowth due to hindered lattice matching with silicon carbide substrate 1can be suppressed. As a result, silicon carbide substrate 1 capable ofimproving yield of a semiconductor device can be provided.

Next, a semiconductor device in the present embodiment will bedescribed.

Referring to FIG. 6, a lateral MESFET (Metal Semiconductor Field EffectTransistor) as a semiconductor device 100 in the present embodimentmainly has a p− type silicon carbide substrate 103 and an n− typesilicon carbide epitaxial growth layer 102. The lateral MESFET includesan n+ type source impurity region 111 and an n+ type drain impurityregion 114 in a region with a certain depth from a main surface of n−type silicon carbide epitaxial growth layer 102 on a side not facing p−type silicon carbide substrate 103 (an upper side in FIG. 6). A sourceelectrode 121 and a drain electrode 124 are formed on upper mainsurfaces of n+ type source impurity region 111 and n+ type drainimpurity region 114, respectively. A gate electrode 122 is formedbetween source electrode 121 and drain electrode 124. An interlayerinsulating film 106 is arranged between source electrode 121 and gateelectrode 122, and between gate electrode 122 and drain electrode 124. Asubstrate back-surface electrode 127 is arranged on a main surface of p−type silicon carbide substrate 103 on a side not facing n− type siliconcarbide epitaxial growth layer 102 (a lower side in FIG. 6). It is notedthat p type and n type of the components described above may bereversed.

For example, p− type silicon carbide substrate 103 is formed of p typesilicon carbide. P− type means having a low p type impurityconcentration, a high resistance, and semi-insulating properties.Specifically, p− type silicon carbide substrate 103 is made of a siliconcarbide substrate having a thickness of not less than 100 μm and notmore than 400 μm, and a concentration of boron atoms as an impurity of1×10¹⁵ cm⁻³. Further, n− type silicon carbide epitaxial growth layer 102is formed of an epitaxial layer having a low n type impurityconcentration. Specifically, n− type silicon carbide epitaxial growthlayer 102 has a thickness of about 1 μm, and a concentration of nitrogenatoms as an impurity of 1×10¹⁷ cm⁻³. In addition, n+ type sourceimpurity region 111 and n+ type drain impurity region 114 are eachformed of an n type injection layer. N+ type means having a high n typeimpurity concentration. Specifically, n+ type source impurity region 111is an about 0.4 μm-thick n type injection layer containing about 1×10¹⁹cm⁻³ of nitrogen atoms. An n− type silicon carbide substrate can containnitrogen as an impurity. A p− type silicon carbide epitaxial growthlayer can contain aluminum as an impurity.

Although the present embodiment has described the MESFET as exemplarysemiconductor device 100, the semiconductor device is not limitedthereto. Semiconductor device 100 may be, for example, a HEMT (HighElectron Mobility Transistor), a lateral JFET (Junction Field EffectTransistor), a lateral MESFET, HFET (Heterojunction Field EffectTransistor), or the like.

Next, one example of a method for manufacturing the lateral MESFET assemiconductor device 100 in the present embodiment will be described.

Referring to FIG. 7, in the method for manufacturing the MESFET in thepresent embodiment, first, a silicon carbide substrate preparation stepis performed as step (S110). In this step (S110), silicon carbidesubstrate 1 described above is prepared. Specifically, silicon carbidesubstrate 1 is prepared in which, in at least one main surface of thesilicon carbide substrate, sulfur atoms are present at not less than60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and carbonatoms as an impurity are present at not less than 3 at % and not morethan 25 at %.

Next, an epitaxial growth step is performed as step (S120).Specifically, silicon carbide epitaxial growth layer 102 is formed onthe one main surface in which the sulfur atoms are present at not lessthan 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and thecarbon, atoms as the impurity are present at not less than 3 at % andnot more than 25 at %.

Next, an ion implantation step is performed as step (S121). In this step(S121), n+ type source impurity region 111 and n+ type drain impurityregion 114 are formed by performing ion implantation on epitaxial growthlayer 102 formed in step (S120).

Next, an activation annealing step is performed as step (S122). In thisstep (S122), for example, heat treatment for heating to about 1600° C.to 1900° C. is performed. Thereby, the impurity implanted in step (S121)is activated.

Next, an electrode formation step is performed as step (S130). In thisstep (S130), substrate back-surface electrode 127 is formed on a side ofsilicon carbide substrate 103 opposite to a side thereof on whichsilicon carbide epitaxial growth layer 102 is formed. Thereby, theMESFET as semiconductor device 100 is completed.

Next, function and effect of semiconductor device 100 and the method formanufacturing semiconductor device 100 in the present embodiment will bedescribed.

In semiconductor device 100 and the method for manufacturingsemiconductor device 100 in the present embodiment, silicon carbidesubstrate 1 having a stabilized surface is used. Thus, high-qualityepitaxial growth layer 102 is formed on silicon carbide substrate 1.Further, formation of a low resistance layer at an interface betweensilicon carbide substrate 1 and epitaxial growth layer 102 can besuppressed. As a result, yield of semiconductor device 100 can beimproved.

Second Embodiment

Next, a silicon carbide substrate in a second embodiment will bedescribed. Referring to FIG. 8, silicon carbide substrate 1 in thesecond embodiment basically has the same structure and exhibits the sameeffect as those of silicon carbide substrate 1 in the first embodiment.However, silicon carbide substrate 1 in the second embodiment isdifferent from that in the first embodiment in that it includes a baselayer 11 and a single-crystal silicon carbide layer 12.

Specifically, referring to FIG. 8, silicon carbide substrate 1 in thesecond embodiment includes base layer 11, and single-crystal siliconcarbide layer 12 formed on base layer 11. Further, a main surface 12A ofsingle-crystal silicon carbide layer 12 on a side opposite to a sidefacing base layer 11 corresponds to main surface 1A in the firstembodiment. That is, in silicon carbide substrate 1 in the presentembodiment, a region including one main surface 12A is made ofsingle-crystal silicon carbide. The presence ratio of sulfur atoms inone main surface 12A is not less than 60×10¹⁰ atoms/cm² and not morethan 2000×10¹⁰ atoms/cm², and the presence ratio of carbon atoms as animpurity in one main surface 12A is not less than 3 at % and not morethan 25 at %.

In silicon carbide substrate 1 in the present embodiment, an inexpensivebase substrate, for example, a substrate made of single-crystal siliconcarbide having a high defect density, a polycrystalline silicon carbidesubstrate, or a base substrate made of ceramics, is adopted as baselayer 11, and a substrate made of a good-quality silicon carbide singlecrystal (tile substrate) is arranged on base layer 11 to serve assingle-crystal silicon carbide layer 12. Thus, silicon carbide substrate1 in the present embodiment serves as a silicon carbide substrate withsuppressed manufacturing cost. Further, in the present embodiment,silicon carbide substrate 1 has a structure in which a plurality ofsingle-crystal silicon carbide layers 12 are arranged on base layer 11with a large diameter, side by side when viewed in plan view. As aresult, silicon carbide substrate 1 in the present embodiment serves asa silicon carbide substrate with suppressed manufacturing cost and witha large diameter.

In other words, silicon carbide substrate 1 in the present embodiment isa composite silicon carbide substrate formed of a strength retentionportion (base layer 11) and a surface portion (tile substrates). Thestrength retention portion of the composite silicon carbide substrate isnot required to be made of single-crystal silicon carbide as long as ithas heat resistance and strength, and it is only necessary that thesurface portion is made of single-crystal silicon carbide. From theviewpoint of heat resistance and strength, the strength retentionportion is preferably made of silicon carbide. The silicon carbideconstituting the strength retention portion may be any of apolycrystalline body produced by vapor-phase growth, a sintered bodymade of an inorganic or organic source material, and a monocrystallinebody. Since the surface portion is epitaxially grown, it is required tobe made of single-crystal silicon carbide.

Next, a method for manufacturing the silicon carbide substrate in thepresent embodiment will be described. Referring to FIG. 9, in the methodfor manufacturing the silicon carbide substrate in the presentembodiment, first, steps (S10) to (S30) are performed as in the firstembodiment. Thereafter, a single-crystal substrate shaping step isperformed as step (S31). In this step (S31), a substrate obtained as aresult of steps (S10) to (S30) is shaped into a shape suitable toconstitute single-crystal silicon carbide layer 12 shown in FIG. 8.Specifically, for example, a plurality of rectangular substrates areprepared by shaping the substrate obtained as a result of steps (S10) to(S30). Next, a bonding step is performed as step (S32). In this step(S32), the plurality of substrates fabricated in step (S31) are arrangedon a separately prepared base substrate, side by side when viewed inplan view, for example, in a matrix state. Thereafter, the substratesfabricated in step (S31) are integrated with the base substrate by beingsubjected to treatment of heating them to a predetermined temperature,and a structural body in which the plurality of single-crystal siliconcarbide layers 12 are arranged on base layer 11, side by side whenviewed in plan view, is obtained, as shown in FIG. 8.

Bonding of base layer 11 and single-crystal silicon carbide layers 12can be performed using close-spaced sublimation or an adhesive. Theadhesive may be any of organic and inorganic adhesives as long as it canretain strength. Further, as the adhesive, a polymer such aspolycarbosilane containing silicon and carbon and forming SiC bondingwhen heated may be used.

Thereafter, steps (S40) to (S60) are performed as in the aboveembodiment, and thereby silicon carbide substrate 1 in the secondembodiment is completed.

Since completed composite silicon carbide substrate 1 has no constraintsin the orientation and size of crystal growth, a substrate with desiredplane orientation and size can be obtained. Further, an inexpensivepolycrystal or sintered body can be used as the base substrate. Inaddition, single-crystal silicon carbide layers 12 can be thinned. Thus,composite silicon carbide substrate 1 having the base substrate andsingle-crystal silicon carbide layers 12 bonded together can bemanufactured at an inexpensive cost when compared with a single-crystalsilicon carbide substrate of the same size.

Example 1

An experiment was conducted to investigate the influence of the presenceratios of sulfur atoms and carbon atoms as an impurity in a main surfaceof a silicon carbide substrate on yield of a semiconductor device.

A silicon carbide single crystal was grown by the sublimation method.Silicon carbide substrate 1 with a diameter of 80 mm was used as a seedsubstrate. The seed substrate had a main surface corresponding to a(0001) plane. A grown surface, an underlying substrate surface, and anouter periphery of the silicon carbide single crystal were ground withan outer periphery grinding machine to obtain an ingot of the siliconcarbide single crystal. Slicing of the ingot was performed using amultiwire saw. The ingot was cut such that a main surface of slicedsilicon carbide substrate 1 (hereinafter also referred to as a surface)would serve as a surface that was 4° off from the (0001) plane. Slicedsilicon carbide substrate 1 had a thickness of 400 μm. Silicon carbidesubstrate 1 had a resistivity of 2×10⁵ Ωcm. After slicing, chamferprocessing was performed on an outer periphery of the substrate. Thesubstrate subjected to the chamfer processing had a diameter of 76.2 mm.Front and back surfaces of the substrate were sequentially flattened toobtain a substrate for epitaxial growth. The back surface was subjectedto grinding processing using a diamond grindstone, and thereaftersubjected to polishing processing to be mirror finished such that thesurface of silicon carbide substrate 1 would have an Rq of 0.3 to 10 nm.An in-feed type grinding machine was used for the grinding processing,and a vitrified bonded grindstone having a mesh size of #2400 and aconcentration degree of 150 was used as the grindstone. In the polishingprocessing, lapping was performed. A tin surface plate was used as asurface plate. Diamond slurry had a grain size of 1 μm.

In order to process the front surface, grinding processing, lappingprocessing, and thereafter CMP were performed. Colloidal silica with anaverage grain size of 30 nm was used as abrasive grains of slurry forthe CMP. In order to improve a rate and control surface composition,sulfuric acid, tartaric acid, and aqueous hydrogen peroxide were addedas chemical components of the slurry. In the present invention'sexample, x and y were adjusted to satisfy the condition of−50x+700≦y≦−50x+1800, where x represents a pH of the slurry, and yrepresents an oxidation-reduction potential.

A suede-type polishing cloth was adopted. Further, resistancecoefficient R (m²/s), which is represented using viscosity η (mPa·s) ofa polishing liquid, liquid flow rate Q (m³/s), area S (m²) of apolishing surface plate, polishing pressure P (kPa), and peripheralspeed V(m/s) (here, R=η×Q×V/S×P), was set to 2.0×10⁻¹⁵ to 8.0×10⁻¹⁴(m²/s) in the present invention's example.

To evaluate the surface composition, the amount of sulfur atoms (S) wasmeasured by TXRF, and the amount of carbon atoms (C) as an impurity wasmeasured by XPS. Devices were produced using substrates (sample numbers1-1 to 1-11) with controlled surface compositions. The produced deviceswere used to manufacture lateral MESFETs. Yields of the MESFETs werecalculated. In the calculation of the yields, a MESFET having a gatecurrent density of 1×10⁻⁶ A/cm² when a gate voltage of 5 V was appliedthereto was determined as good. Table 1 shows results of the yields ofthe MESFETs manufactured by changing the presence ratios of the sulfuratoms and the carbon atoms as the impurity in the main surface ofsilicon carbide substrate 1.

TABLE 1 Sample Unit 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 1-10 1-11 S×10¹⁰ atoms/cm² 45 45 60 60 120 400 800 2000 2000 2500 2500 C at % 2 3 23 7 15 21 25 30 25 30 Device yield % 37 42 44 61 68 74 63 54 35 32 27

Sample numbers 1-4 to 1-8 are MESFETs in accordance with the presentinvention, and the others are MESFETs in accordance with comparativeexamples. In silicon carbide substrate 1 forming the MESFETs of thepresent invention's examples, the sulfur atoms are present in thesurface of the substrate at not less than 60×10¹⁰ atoms/cm² and not morethan 2000×10¹⁰ atoms/cm², and the carbon atoms as the impurity arepresent in the surface of the substrate at not less than 3 at % and notmore than 25 at %. As shown in Table 1, it was confirmed that theMESFETs of the present invention's examples achieve better yields thanthose of the MESFETs of the comparative examples.

Example 2

An experiment was conducted to investigate the influence of the presenceratio of chlorine atoms in a main surface of a silicon carbide substrateon yield of a semiconductor device.

Surface composition was controlled by changing conditions for the CMP.In order to improve a rate and control the surface composition, sodiumsulfate, sodium malate, and sodium dichloroisocyanurate were added aschemical components of slurry. Colloidal silica with an average grainsize of 50 nm was used as abrasive grains of the slurry for the CMP. Asuede-type polishing cloth was used. Further, in the present invention'sexample, resistance coefficient R (m²/s) was set to a range of 3.0×10⁻¹⁵to 8.0×10⁻¹⁵ (m²/s). In the present invention's example, x and y werecontrolled to satisfy the condition of −50x+1100≦y≦−50x+1800, where xrepresents a pH of the slurry, and y represents an oxidation-reductionpotential. Other conditions are the same as those in Example 1.

Lateral MESFETs were produced using substrates (samples 2-1 to 2-5) withcontrolled surface compositions, and yields of the MESFETs werecalculated as in Example 1. Table 2 shows results of the yields of theMESFETs manufactured by changing the presence ratio of the chlorineatoms in the main surface of silicon carbide substrate 1.

TABLE 2 Sample Unit 2-1 2-2 2-3 2-4 2-5 S ×10¹⁰ atoms/cm² 900 900 900900 900 C at % 20 20 20 20 20 Cl ×10¹⁰ atoms/cm² 100 900 1300 3000 4000Device yield % 69 66 64 62 57

Sample numbers 2-1 to 2-4 are MESFETs in accordance with the presentinvention, and sample number 2-5 is a MESFET in accordance with acomparative example. In silicon carbide substrate 1 forming the MESFETsof the present invention's examples, the chlorine atoms are present inthe surface of the substrate at not more than 3000×10¹⁰ atoms/cm². Asshown in Table 2, it was confirmed that the MESFETs of the presentinvention's examples achieve better yields than that of the MESFET ofthe comparative example.

Example 3

An experiment was conducted to investigate the influence of the presenceratio of oxygen atoms in a main surface of a silicon carbide substrateon yield of a semiconductor device.

Surface composition was controlled by changing conditions for the CMP.In order to improve a rate and control the surface composition, sodiumhydrogen sulfate, sodium carbonate, TMAH, and aqueous hydrogen peroxidewere added as chemical components of slurry. Colloidal silica with anaverage grain size of 50 nm was used as abrasive grains of the slurryfor the CMP. A suede-type polishing cloth was used. In the presentinvention's example, resistance coefficient R (m²/s) was set to a rangeof 3.0×10⁻¹⁵ to 8.0×10⁻¹⁵ (m²/s). In the present invention's example, xand y were controlled such that an oxidation-reduction potentialsatisfies the condition of −50x+700≦y≦−50x+1100, where x represents a pHof the slurry, and y represents the oxidation-reduction potential. Otherconditions are the same as those in Example 1.

Lateral MESFETs were produced using substrates (samples 3-1 to 3-8) withcontrolled surface compositions, and yields of the MESFETs werecalculated as in Example 1. Table 3 shows results of the yields of theMESFETs manufactured by changing the presence ratio of the oxygen atomsin the main surface of silicon carbide substrate 1.

TABLE 3 Sample Unit 3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 S ×10¹⁰ atoms/cm²800 800 800 800 800 800 800 800 C at % 22 22 22 22 22 22 22 22 O at % 23 5 9 15 21 30 35 Device yield % 60 63 65 69 68 64 62 58

Sample numbers 3-2 to 3-7 are MESFETs in accordance with the presentinvention's examples, and sample numbers 3-1 and 3-8 are MESFETs inaccordance with comparative examples. In silicon carbide substrate 1forming the MESFETs of the present invention's examples, the oxygenatoms are present in the surface of the substrate at not less than 3 at% and not more than 30 at %. As shown in Table 3, it was confirmed thatthe MESFETs of the present invention's examples achieve better yieldsthan those of the MESFETs of the comparative examples.

Example 4

An experiment was conducted to investigate the influence of the presenceratio of a metal impurity in a main surface of a silicon carbidesubstrate on yield of a semiconductor device.

Surface composition was controlled by changing conditions for the CMP.In order to improve a rate and control the surface composition, sodiumsulfate, sodium malate, and sodium dichloroisocyanurate were added aschemical components of slurry. Colloidal silica with an average grainsize of 50 nm was used as abrasive grains of the slurry for the CMP. Asuede-type polishing cloth was used. Further, in the present invention'sexample, resistance coefficient R (m²/s) was set to a range of 3.0×10⁻¹⁵to 8.0×10⁻¹⁵ (m²/s). In the present invention's example, x and y werecontrolled such that an oxidation-reduction potential satisfies thecondition of −50x+1100≦y≦−50x+1800, where x represents a pH of theslurry, and y represents the oxidation-reduction potential. Otherconditions are the same as those in Example 1.

Lateral MESFETs were produced using substrates (samples 4-1 to 4-5) withcontrolled surface compositions, and yields of the MESFETs werecalculated as in Example 1. Table 4 shows results of the yields of theMESFETs manufactured by changing the presence ratio of the metalimpurity in the main surface of silicon carbide substrate 1.

TABLE 4 Sample Unit 4-1 4-2 4-3 4-4 4-5 S ×10¹⁰ atoms/cm² 70 70 70 70 70C at % 3 3 3 3 3 Metal ×10¹⁰ atoms/cm² 9 80 900 4000 5000 Device yield %70 67 66 64 62

Sample numbers 4-1 to 4-4 are MESFETs in accordance with the presentinvention's examples, and sample number 4-5 is a MESFET in accordancewith a comparative example. In silicon carbide substrate 1 forming theMESFETs of the present invention's examples, the metal impurity ispresent in the surface of the substrate at not more than 4000×10¹⁰atoms/cm². As shown in Table 4, it was confirmed that the MESFETs of thepresent invention's examples achieve better yields than that of theMESFET of the comparative example.

Example 5

An experiment was conducted to investigate the influence of a surfaceroughness of a main surface of a silicon carbide substrate on yield of asemiconductor device.

In the present example, a silicon carbide substrate with a diameter of125 mm was used. Surface composition was controlled by changingconditions for the CMP. Colloidal silica with an average grain size of20 to 100 nm was used as abrasive grains of slurry for the CMP. Asuede-type polishing cloth was used. Further, in the present invention'sexample, resistance coefficient R (m²/s) was set to 2.0×10⁻¹⁵ to5.0×10⁻¹⁵ (m²/s). In the present invention's example, x and y wereadjusted to satisfy the condition of −50x+700≦y≦−50x+1100, where xrepresents a pH of the slurry, and y represents an oxidation-reductionpotential. Other conditions are the same as those in Example 1.

Lateral MESFETs were produced using substrates (samples 5-1 to 5-4) withcontrolled surface compositions, and yields of the MESFETs werecalculated as in Example 1. Table 5 shows results of the yields of theMESFETs manufactured by changing the surface roughness of the mainsurface of silicon carbide substrate 1.

TABLE 5 Sample Unit 5-1 5-2 5-3 5-4 S ×10¹⁰ atoms/cm² 60 60 60 60 O at %4 4 4 4 Roughness nm 0.1 0.3 0.5 1 Device yield % 71 70 68 64

Sample numbers 5-1 to 5-3 are MESFETs in accordance with the presentinvention's examples, and sample number 5-4 is a MESFET in accordancewith a comparative example. In silicon carbide substrate 1 forming theMESFETs of the present invention's examples, the surface of thesubstrate has a surface roughness of not more than 0.5 nm when evaluatedin Rq. As shown in Table 5, it was confirmed that the MESFETs of thepresent invention's examples achieve better yields than that of theMESFET of the comparative example.

Example 6

An experiment was conducted to investigate the influence of a planeorientation of a main surface of a silicon carbide substrate on yield ofa semiconductor device.

A substrate was fabricated to have a main surface corresponding to the(000-1) plane. The single-crystal substrate had a diameter of 110 mm. Inorder to control surface composition and roughness to be withinappropriate ranges, colloidal silica with a grain size of 10 nm wasused, and resistance coefficient R (m²/s) in the present invention'sexample was set to 5.0×10⁻¹⁴ to 8.0×10⁻¹⁴ (m²/s). In the presentinvention's example, x and y were controlled such that anoxidation-reduction potential satisfies the condition of−50x+700≦y≦−50x+1000, where x represents a pH of slurry, and yrepresents the oxidation-reduction potential. Other conditions are thesame as those in Example 1. MESFETs were produced using silicon carbidesubstrate 1 having the main surface corresponding to the (000-1) plane.

Table 6 shows results thereof. Sample numbers 6-3 to 6-8 are MESFETs inaccordance with the present invention's examples, and the others areMESFETs in accordance with comparative examples. It was confirmed thatthe MESFETs of the present invention's examples, which use siliconcarbide substrate 1 having a surface in which the sulfur atoms arepresent at not less than 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰atoms/cm², and the carbon atoms as the impurity are present at not lessthan 3 at % and not more than 25 at %, achieve better yields than thoseof the MESFETs using substrates having surface compositions of thecomparative examples.

TABLE 6 Sample Unit 6-1 6-2 6-3 6-4 6-5 6-6 6-7 6-8 6-9 6-10 S ×10¹⁰atoms/cm² 40 60 60 80 120 600 800 2000 2000 2700 C at % 3 2 3 7 10 18 2125 29 25 Device yield % 41 45 60 62 70 71 64 53 32 36

Example 7

An experiment was conducted to investigate the influence of a planeorientation of a main surface of a silicon carbide substrate on yield ofa semiconductor device.

A silicon carbide single crystal was grown by the sublimation method. Asilicon carbide substrate with a diameter of 80 mm was used as a seedsubstrate. The seed substrate had a main surface corresponding to the(0001) plane. A grown surface, an underlying substrate surface, and anouter periphery of the silicon carbide single crystal were ground withan outer periphery grinding machine to obtain an ingot of siliconcarbide. Slicing was performed using a multiwire saw. In order to obtaina sliced substrate surface corresponding to {03-38}, the ingot was setin a wire saw apparatus with being tilted by 54.7° from a runningdirection of a wire, and was cut. A sliced substrate had a thickness of250 μm. An outer periphery of the sliced substrate was diced to obtaintile substrates of 20 mm×30 mm.

Polycrystalline silicon carbide was grown by the sublimation method. Aningot with a diameter of 155 mm was obtained by outer peripheryprocessing. The ingot was sliced with a multiwire saw to obtain apolycrystalline substrate with a thickness of 500 μm. The rectangularsingle-crystal substrates were arranged on the polycrystallineunderlying substrate, and bonded by close-spaced sublimation. An outerperiphery of the bonded composite substrate was processed to obtain asubstrate with a diameter of 150 mm and a thickness of 750 μm. In thepresent invention's example, x and y were adjusted to satisfy thecondition of −50x+700≦y≦−50x+1100, where x represents a pH of slurry,and y represents an oxidation-reduction potential. Further, in thepresent invention's example, resistance coefficient R (m²/s) was set to5.0×10⁻¹⁵ to 1.0×10⁻¹⁴ (m²/s).

Other conditions are the same as those in Example 1. MESFETs wereproduced using silicon carbide substrate 1 having the main surfacecorresponding to the (03-38) plane.

Table 7 shows results thereof. Sample numbers 7-2 to 7-5 are MESFETs inaccordance with the present invention's examples, and the others areMESFETs in accordance with comparative examples. It was confirmed thatthe MESFETs of the present invention's examples, which use siliconcarbide substrate 1 having a surface in which the sulfur atoms arepresent at not less than 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰atoms/cm², and the carbon atoms as the impurity are present at not lessthan 3 at % and not more than 25 at %, achieve better yields than thoseof the MESFETs using substrates having surface compositions of thecomparative examples.

TABLE 7 Sample Unit 7-1 7-2 7-3 7-4 7-5 7-6 S ×10¹⁰ atoms/cm² 40 60 300500 2000 2600 C at % 2 3 12 16 25 30 Device yield % 36 62 75 73 55 31

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A silicon carbide substrate, comprising: a firstmain surface; and a second main surface opposite to said first mainsurface, a region including at least one main surface of said first andsecond main surfaces consisting essentially of single-crystal siliconcarbide, sulfur atoms being present in said one main surface at not lessthan 60×10¹⁰ atoms/cm² and not more than 2000×10¹⁰ atoms/cm², and carbonatoms as an impurity being present in said one main surface at not lessthan 3 atomic percent (at %) and not more than 25 atomic percent (at %).2. The silicon carbide substrate according to claim 1, wherein chlorineatoms are present in said one main surface at not more than 3000×10¹⁰atoms/cm².
 3. The silicon carbide substrate according to claim 1,wherein oxygen atoms are present in said one main surface at not lessthan 3 atomic percent (at %) and not more than 30 atomic percent (at %).4. The silicon carbide substrate according to claim 1, wherein a metalimpurity is present in said one main surface at not more than 4000×10¹⁰atoms/cm².
 5. The silicon carbide substrate according to claim 1,wherein said one main surface has a surface roughness of not more than0.5 nm when evaluated in Rq.
 6. The silicon carbide substrate accordingto claim 1, having a diameter of not less than 110 mm.
 7. The siliconcarbide substrate according to claim 1, having a diameter of not lessthan 125 mm and not more than 300 mm.
 8. The silicon carbide substrateaccording to claim 1, wherein said single-crystal silicon carbide has a4H structure, and said one main surface has an off angle of not lessthan 0.1° and not more than 10° relative to a {0001} plane of saidsingle-crystal silicon carbide.
 9. The silicon carbide substrateaccording to claim 1, wherein said single-crystal silicon carbide has a4H structure, and said one main surface has an off angle of not morethan 4° relative to a {03-38} plane of said single-crystal siliconcarbide.
 10. The silicon carbide substrate according to claim 1,comprising: a base layer; and a single-crystal silicon carbide layerformed on said base layer, wherein said one main surface is a surface ofsaid single-crystal silicon carbide layer on a side opposite to a sidefacing said base layer.
 11. The silicon carbide substrate according toclaim 1, wherein each concentration of sulfur atoms and carbon atoms isadjusted so that silicon is suppressed from adhering to said one mainsurface.